Source of Infrared Light

The pigments discussed in this chapter are desirable because of the reflective properties exhibited in the infrared region of the electromagnetic spectrum. Before discussing the pigments, a short treatment of the background physics is in order. 

The light intensity at any given wavelength is a function of temperature only, h, c, and k are all physical constants. This equation can be used for blackbody objects 

---

It is ironic to consider the III-V nitrides, the premier materials for short wavelength blue and UV emitters, as sources of infrared light. However, Er-doped GaN is of interest for making electrically pumped, temperature insensitive, broad band and compact optical amplifiers or sources of 1.54 pm light. Applications include long-haul communication systems (amplifiers), local area networks (50/50 splitters) and sources (lasers) for transmission in silica-based optical fibres. 

The most frequently used source of infrared light for infrared spectrometers is so called the Nemst stick. This stick is about two to four centimeters long and one to three millimeters thick, and is made from zirconium oxide with additions of yttrium oxide and oxides of other metals. This mixture of oxides has a negative temperature coefficient of electrical resistance. This means that its electrical conductivity increases with an increase in temperature. At room temperature, the Nemst stick is a non-conductor. Thus, an auxiliary heating is necessary for ignition of the Nernst stick. Even if the Nernst stick is red-hot, it can be heated further by electricity. The normal operating temperature of this infrared light source is approximately 1900 K. 

Optical Measurements. The methods of Hamill et ah (8, 9,18) were employed without any significant modification. Spectra were recorded against air as a reference. The near infrared spectrum of the trapped electron was then plotted as the difference between the two spectra obtained before and after bleaching the sample with an intense source of infrared light (A > 1000 n.m.). Other details will be given with the results. 

To illustrate infrared absorption, we require a simple IR detector connected to a voltmeter (a suitable detector is the infrared sensor , sold by the Philip Harris Company). There is usually sufficient IR radiation reflected by benches and walls to provide a substantial background laboratory source of infrared light (Fig. 20.13(a)). 

The spectrometer consists of a source of infrared light, emitting radiation throughout the whole frequency range of the instrument. Light from the source is split into two beams of equal intensity. One beam is made to pass through the sample while the other is allowed to behave as the reference beam. The function of such a double beam operation is to measure the difference in Intensities between the two beams at each wavelength. 

So we cannot see infrared light even when looking at photons coming off a heat source because photons of infrared light do not interact with the chemicals at the back of the eye (see later example), unlike photons of visible light. 

The heart of a Fourier transform infrared spectrophotometer is the interferometer in Figure 20-26. Radiation from the source at the left strikes a beamsplitter, which transmits some light and reflects some light. For the sake of this discussion, consider a beam of monochromatic radiation. (In fact, the Fourier transform spectrophotometer uses a continuum source of infrared radiation, not a monochromatic source.) For simplicity, suppose that the beamsplitter reflects half of the light and transmits half. When light strikes the beamsplitter at point O, some is reflected to a stationary mirror at a distance OS and some is transmitted to a movable mirror at a distance OM. The rays reflected by the mirrors travel back to the beamsplitter, where half of each ray is transmitted and half is reflected. One recombined ray travels in the direction of the detector, and another heads back to the source. 

Formulations similar to those used in Dry Silver but without the AgX component are light stable but can be imaged thermally. The silver behenate develops to form a black silver image where (and only where) heat is applied. Materials of this type were originally intended for reflex exposure. Here the film is sandwiched with a document and exposed to a source of infrared radiation. The ink on the document absorbs the infrared and converts it to heat, which initiates development of the film, where it is in contact with the ink image on the paper. Overhead projector transparencies, sometimes called view graphs, ... 

Stray light Stray light is dealt with in other sections of this book however, the main source of stray light (also termed stray radiant energy) within the IR microscope is the spurious energy caused by diffraction of infrared energy through small apertures. 

A synchrotron source of infrared radiation is required to penetrate the hydrated Micrasterias cell, which is about 12 pm thick. Under these conditions, information can be obtained reliably from all spectral regions except the amide I band. Because of the light scattering that occurs through the cell body, spatial resolution is limited to about 20 pm. This is still adequate to differentiate cellular regions containing chloroplasts, the nuclear region, and cytoplasmic areas devoid of chloroplast or nucleus (4). 

In a dispersive VCD spectrometer, the IR source in Figure 2 consists of a thermal or arc source of infrared radiation, a light chopper, and a grating monochromator. The infrared source of radiation is first... 

In the most general temis, an infrared spectrometer consists of a light source, a dispersmg element, a sample compartment and a detector. Of course, there is tremendous variability depending on the application. 

---